Technical Note pubs.acs.org/ac
New Insight into a Microfluidic-Based Bipolar System for an Electrochemiluminescence Sensing Platform Xiaowei Zhang,†,‡ Chaogui Chen,†,‡ Jing Li,† Libing Zhang,† and Erkang Wang*,† †
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China S Supporting Information *
ABSTRACT: In this work, a novel style of a microfluidic-based bipolar system with twodirection driving electrodes and dual-channel configuration was described for the first time, which could reach 100% current efficiency in theory. More importantly, the background signal from the integrated driving electrodes was completely eliminated, when this unique design was used to construct an electrochemiluminescence (ECL) sensing platform. First, universal pH indicator was employed to study the mechanism and demonstrate that this new bipolar system possessed 100% current efficiency theoretically. Then, the Ru(bpy)32+/TPrA ECL system was introduced to construct the dual-channel bipolar ECL sensing platform, and the results of visual ECL experiments proved that the background signals from the driving electrodes were completely dispelled with our design. To illustrate the promising applications of this dualchannel device, TPrA, dopamine (DA), H2O2, and K3Fe(CN)6 were detected as model targets under different principles.
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the separation of analytes and the ECL probe, Crooks and coworkers developed a dual-channel bipolar device.8 Very recently, a closed BPE system has been reported, which highly improved the ηc of BPE system.24 However, most of the works could hardly solve all the problems described above perfectly, and thus, more efforts should be made to improve the performance of the bipolar system. Herein, a microfluidic ECL sensing platform based on a novel style of the bipolar system was presented for the first time (Scheme 1). Owing to the use of the two-direction driving electrode (Scheme 1, a driving electrode, such as driving anode or driving cathode, which is inserted into a channel from the two ends of this channel and possesses only one potential) and dual-channel mode, the ηc of our design was improved to 100% in theory. Moreover, the enormous background ECL signal from the driving electrode was eliminated completely from the origin. In the following parts, the mechanism, ECL behavior, and potential distribution of our design have been investigated in detail. Results showed that this design effectively solved all the present problems in bipolar systems mentioned above. The applicability of the device was successfully demonstrated by detecting TPrA (coreactant), dopamine (DA, quencher), H2O2, and K3Fe(CN)6 (electroactive analyte) with good performance. Thus, we believe that the proposed bipolar ECL sensing platform is attractive in the real-time analysis of various targets.
bipolar electrode (BPE) is an electronic conductor, which can act as both anode and cathode without any direct electrical connection with the power supply.1 As it is easy to be fabricated, integrated, and controlled, the BPE system has been adopted as a useful tool in chemistry, such as analysis,2−8 synthesis,9−14 analytes separation and enrichment,15−21 and screening of electrocatalysts.22 Electrochemiluminescence (ECL)23 as a powerful analytical technique has been widely applied in the sensing devices based on the BPE system.2−8 Manz and co-workers first introduced the Ru(bpy)32+ ECL system to the BPE system as the signal reporting method.2 Using this approach, they could detect the ECL coreactants and quenchers. Later, Chang et al. expanded this concept to any electroactive analyte by making full use of both poles of the BPE.3 Although each method exhibited good analytical performances, the BPE system still faces huge challenges. (i) The current efficiency (ηc, ηc = the current flow through the BPE/the current flow through the driving electrodes) of BPE systems was still very low. (ii) The driving anode (cathode) and BPE anode (cathode) were placed in the same channel, thus analytes were preferentially consumed at the driving electrode. Moreover, this layout led to the overwhelming background signal from the driving electrode. (iii) A pair of external driving electrodes with high resistance was always employed to decrease (far from eliminating) the background signal, and this greatly hindered the miniaturization of the microfluidic device. (iiii) The analytes and Ru(bpy)32+ probe were often mixed in the same solvent during analysis. To overcome the drawbacks, several methods have been developed. For example, Chen’s group immobilized Ru(bpy)32+ ECL probe on the surface of a BPE and successfully eliminated the background signal from the driving electrode.5 To achieve © XXXX American Chemical Society
Received: March 18, 2013 Accepted: May 1, 2013
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Visual ECL Experiment. Three different chips (with the same size of driving electrodes, BPE, and channels, one for single-channel design, one for reported dual-channel design, and another for our design) were used in visual ECL experiments (Figure 2-1I, -2I, and -3I, details in Supporting Information). The ECL solution (1 mM Ru(bpy)32+ and 1 mM TPrA in 0.1 M PB pH 7.4) was injected into the devices by a LSP02-1B Syringe pump (Shenzhen, China). The driving voltage was changed from 0 V to be large enough during the experiment. A FUJIFILM JV 155 digital camera was used to record the visual ECL from the electrodes in a darkroom. ECL Measurement. ECL signals were obtained by a model MPI-A capillary electrophoresis ECL system (Xi’an Remax Electronics Co. Ltd.) at room temperature. The driving voltage applied between the two channels was generated by the electrochemical workstation integrated in the same instrument. The sampling rate was set at 100 T/s to achieve the highest ECL stability.
Scheme 1. Fundamental Principle of the Dual-Channel Bipolar ECL Sensor
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RESULTS AND DISCUSSION Demonstration of Principle. A universal pH indicator was employed to illustrate the function of the BPE across the two completely separated reservoirs (Figure 1a).2 As shown in Figure 1b, the electrolysis of water did not occur if there was not any connection between the two reservoirs, and the expected pH changes took place in another device with a BPE as the connection under the same conditions due to the electrolysis of water (Figure 1c). During the whole process, Pt cathode accepted electron from the external power supply, and reduction of H2O occurred (eq 1). At the same time, H2O was oxidized at the BPE anode (eq 2) according to the principle of electric neutrality. Then, electron flowed from the anode to the cathode of the BPE19 and finally arrived at the positive pole of the power supply to form a big loop with a reverse procedure (eqs 3 and 4) of the left reservoir. The results indicated that all the reactions occurring in this device were closely related to each other with the BPE as a connection, which was in agreement with recent research.24 According to the principle of electric neutrality, the amount of reaction that occurs at the Pt cathode (anode) and the BPE anode (cathode) must stay the same. Also, the amount of reaction that occurs at the BPE anode should be equal to that of the BPE cathode in this “connected in series” design. In other words, the ηc of our design reaches 100% in theory. When this construction is used for the ECL system, the light intensity will be closely related to the current through the BPE (demonstrated below). In addition, the driving anode (cathode) was separated from the BPE anode (cathode) in this design, and therefore, the background signal from the driving electrode was eliminated completely. It should be noticed that the dissolved oxygen might be reduced at the cathodes (eq 5), which might further improve the sensitivity of the detection of amine using Ru(bpy)32+ as the ECL reporting probe.23 Pt cathode:
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EXPERIMENTAL SECTION Chemicals and Reagents. All the chemicals used were of analytical reagent grade without any further purification. Ru(bpy)3Cl2·6H2O, TPrA, and DA were purchased from Sigma−Aldrich Chemical Co. (Milwaukee, WI). Universal pH indicator was purchased from J&K Chemical Co. (Beijing, China). The standard stock solutions of TPrA and H2O2 were prepared in the 0.1 M phosphate buffer (PB, pH 7.4), and DA was prepared in the 0.1 M PB (pH 8.6).25 Device Fabrication. AZ50XT photoresist mold and microelectrodes were prepared using standard photolithographic techniques according to the previously published procedure.26 Briefly, microfluidic channels were fabricated from poly(dimethylsiloxane) (PDMS) with the mold prepared. ITO BPE with integrated Au driving electrodes was prepared by a two-procedure photolithography. Finally, the PDMS channel and glass were exposed to air plasma (60 w, 800 μL/ min) for 90 s and then bonded together at 80 °C for 1 h. Mechanism Demonstration. A chip with two reservoirs (2 cm long, 0.8 cm wide, and 0.5 cm deep), connected by an ITO BPE, was used to study the mechanism of the dualchannel BPE system (Figure 1a). 550 μL of universal pH indicator (pH 7.4) and 50 μL of 0.1 M K2SO4 (supporting electrolyte) were added into the reservoirs. The driving voltage between the two Pt electrodes was set to 5 V by a PS-3005D dc power supply (Shenzhen, China).
4H 2O + 4e− → 2H 2 + 4OH−
(1)
BPE anode: 2H 2O − 4e− → O2 + 4H+
Figure 1. (a) Working schematic diagram of the BPE in the dualreservior chip; (b) device without a BPE under the driving voltage of 5 V for 5 min; (c) and (d) device with a BPE under the driving voltage of 5 V for 30 s and 5 min.
(2)
BPE cathode: 4H 2O + 4e− → 2H 2 + 4OH− B
(3)
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Figure 2. Comparison between the reported publications and our work (1: single channel design; 2: reported dual-channel design; 3: our design). (I) Schematic of the device. (II) The potential distribution of this design. (III) The visual ECL behavior from this design under different driving voltages.
than that of the BPE, analytes might be consumed preferentially at the driving electrodes. This further proved that the ηc was very low in these designs. It should also be noted that the potential distribution of these two designs was essentially the same (Figure 2-1-II and -2-II). The ECL behavior of our design was investigated with an Au BPE array consisting of 9 BPEs, and this chip was also applied to study the spatial distribution of the potential (Scheme S1, Supporting Information). In our design, a two-direction driving electrode was employed to improve the performance of the dual-channel BPE system. A two-direction driving electrode means that only one kind of driving electrode, such as driving anode or driving cathode, is inserted into a channel from the two ends of this channel. The channel which was injected with ECL solution (0.1 M PB pH 7.4) was named the reporting channel (supporting channel). Then, the potential of the driving electrode in the supporting (reporting) channel was held positively (negatively), and the corresponding electrode was chosen as the driving anode (cathode). This creative design was more advantageous over the reported designs. First, the two channels (or all the electrodes, Figure 2-3-I) were connected in series by the BPE across them, so the ηc of this design was very high (100% in theory). Second, the driving anode and the anode of BPE were located in two separated channels, and the driving anode was in the channel without any ECL probe. That was to say, there would not be any ECL generated from the driving anode under any conditions, and this further prevented the consumption of the analytes at the
Pt anode: −
2H 2O − 4e → O2 + 4H
+
(4)
O2 reduction: 2H 2O + O2 + 4e− → 4OH−
(5)
Visual ECL Experiment. In this section, the advantages of our design were expounded by comparing the ECL behaviors of the reported designs2,3,8 with ours. Owing to the higher stability and reflectivity of Au, Au BPE was chosen for the visual ECL experiments. As displayed in Figure 2, the overwhelming background signal from the driving electrodes was observed with the previous designs including single-channel (Figure 2-1) and dual-channel designs (Figure 2-2). In the single-channel design, there was not any visual ECL if the driving voltage was set below 2.5 V (Figure 2-1-III). When the voltage was up to 3−3.5 V, the visual ECL appeared only at the driving anode (Figure 2-1-IIIb,c). With the increase of the driving voltage (>4 V), the visual ECL at the anode of the BPE came into being (Figure 2-1-IIId−f). However, much higher background ECL signal emitted from the driving anode made the signal from the BPE negligible. When it came to the reported dual-channel design, the ECL signal from the BPE anode was not obvious even when the driving voltage reached 26 V, while the huge background ECL was observed from the driving anode (Figure 2-2-III). As is known to all, background signal will greatly affect the sensitivity and detection limit of analysis. Furthermore, due to the higher voltage applied between the driving electrodes C
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driving electrodes. Figure 2-3-III showed the expected result. Third, the light emitting area was the entire part of the BPE in the reporting channel, while only the tip of the BPE worked in the single-channel design and no obvious visual ECL was exhibited in the reported dual-channel design (Figure 2-1-III and -2-III). This confirmed that our device exhibited much higher ECL efficiency than the reported designs described above, and this may further improve the sensitivity of analysis. From this experiment, it could also be inferred that the length of the BPE was not related to the potential difference between two ends of the BPE (details in the Supporting Information). This was completely different from single-channel designs of the bipolar system,1,3 in which there was a liner relationship between the BPE length and the fraction of the driving voltage dropped across the BPE. Finally, there was not any visual ECL observed from the part of BPE in the supporting channel during the whole experiment. This indicated that a mixture of fluids did not take place at the BPEs. Therefore, this design could be applied to mix-free or multiphase analysis in the supporting channel. Analytes Detection. To demonstrate the potential applications of our design, detection of four model targets was performed with a chip containing an eccentric ITO BPE (Figure 3I) under the optimized conditions (details in the Supporting Information). Here, the eccentric BPE design was employed to achieve the lowest driving voltage according to eq S1 (Supporting Information).
TPrA and DA were employed as model targets to prove the sensing function of the reporting channel. ECL solutions with different TPrA concentrations were injected into the reporting channel successively in this experiment. As shown in Figure 3IIa, the increased ECL signals were directly related to the concentration of TPrA. A wide linear relationship between ECL intensity and logarithmic TPrA concentration from 0.1 μM to 1 mM (R = 0.9920) was found, and the detection limit of TPrA is 0.1 μM by using the signal (S) to noise ratio (N) S/N = 3. When it came to DA, the ECL was quenched by the oxidation products of DA, and the ECL signal decreased. A linear relationship between ECL intensity and logarithmic DA concentration from 1 nM to 10 μM (R = 0.9921) was obtained with the detection limit down to 0.2 nM (S/N = 3, Figure 3IIb), which was much better than the reported design (linearity range: 1 μM to 1 mM; detection limit: 1 μM).27 These results manifested that all the analytes related to the ECL process could be detected with our device. To illustrate the sensing function of the supporting channel, H2O2 and K3Fe(CN)6 were detected in this channel. It was proved that the increased ECL signals were related to the concentration of H2O2. As depicted in Figure 3IIc, the linear relationship between ECL intensity and logarithmic H2O2 concentration was from 5 μM to 0.1 mM (R = 0.9975), and the detection limit was 2.5 μM (S/N = 3). As to K3Fe(CN)6, the phenomenon was similar. It was found that the linearity range of our design (0.05−5 mM) was considerable compared to the reported dual-channel design (0.1−5 mM),8 and the detection limit of our design (0.04 mM) for K3Fe(CN)6 was also lower than that of it (0.32 mM). Results showed that the electroactive analytes, which were not directly related to the ECL reaction, were allowed to be detected by taking advantage of the supporting channel, and this also proved that our design was promising in the mix-free or multiphase analysis.
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CONCLUSIONS A novel microfluidic ECL sensing platform based on a new style of bipolar system has been presented for the first time. Due to the use of two-direction driving electrodes and the dualchannel mode, our design presented a much higher ηc and ECL efficiency than any other reported designs. In addition, the huge background signal from the driving anode was completely eliminated in our device. With this design, TPrA, DA, H2O2, and K3Fe(CN)6 were detected with good performance, and results demonstrated that our device was promising in the realtime analysis of any analytes that were directly related to the ECL process and mix-free analysis of any electroactive analytes. In summary, this microfluidic bipolar sensing platform with the novel style of bipolar system proposed holds promising potential for designing electrochemical or ECL devices with high integration, high automation, and high throughput. The designed device can be applied to environmental monitoring and clinical diagnosis with bright prospect. Research is in progress.
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ASSOCIATED CONTENT
* Supporting Information S
Figure 3. (I) Configuration of the analysis chip with an eccentric ITO BPE; (II) The ECL response of the sensing interface for the different analytes. (a) TPrA; (b) DA; (c) H2O2; (d) K3Fe(CN)6. Inset: the linear relationship between analyte concentration and ECL intensity.
Additional information about the assay optimization and potential distribution. This material is available free of charge via the Internet at http://pubs.acs.org. D
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Technical Note
(26) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27− 40. (27) Zhan, W.; Alvarez, J.; Sun, L.; Crooks, R. M. Anal. Chem. 2003, 75, 1233−1238.
AUTHOR INFORMATION
Corresponding Author
*Tel: +86-431-85262003. Fax: +86-431-85689711. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China with Grant Nos. 21190040, 21075120, and 21105094 and the 973 project 2010CB933600 and 2009CB930100.
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